What is the role of EMG/NCS in the workup of Charcot-Marie-Tooth disease (CMT)?

Updated: Jun 23, 2021
  • Author: Divakara Kedlaya, MBBS; Chief Editor: Elizabeth A Moberg-Wolff, MD  more...
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Answer

Electromyography/nerve conduction study (EMG/NCS) [5, 6, 7, 8, 9, 10]

  • If Charcot-Marie-Tooth disease (CMT) is suggested, perform an EMG/NCS first. Findings vary depending on the type of CMT.

  • In demyelinating types of CMT, such as CMT-1, diffuse and uniform slowing of nerve conduction velocities is observed (see following images).

    Nerve conduction study showing decreased nerve con Nerve conduction study showing decreased nerve conduction velocity in the median nerve in an 18-year-old woman with Charcot-Marie-Tooth disease type 1.
    Right ulnar motor nerve conduction study in a 29-y Right ulnar motor nerve conduction study in a 29-year-old patient with advanced Charcot-Marie-Tooth disease type 1.
    Left ulnar motor nerve conduction study in 29-year Left ulnar motor nerve conduction study in 29-year-old patient with advanced Charcot-Marie-Tooth disease type 1.
  • Harding and Thomas criteria for diagnosing CMT-1 include median motor nerve conduction velocity of less than 38 meters per second (m/s), with compound motor action potential (CMAP) and amplitude of at least 0.5 millivolts (mV). No focal conduction block or slowing should be present unless associated with other focal demyelinating processes.

  • All sensory and motor nerves that are tested show the same degree of marked slowing.

  • Absolute values vary, but they are approximately 20-25 m/s in CMT-1 and less than 10 in Dejerine-Sottas disease and congenital hypomyelination. Slowing of nerve conduction can also be found in asymptomatic individuals. In X- linked CMTs, motor nerve conduction velocities are faster than in CMT1A. [68]

  • In neuronal (ie, axonal) types of CMT, nerve conduction velocity usually is normal, but markedly low amplitudes are noted in sensory (ie, sensory nerve action potential [SNAP]) and motor (ie, CMAP) nerve studies.

  • In neuronal (ie, axonal) types of CMT, increased insertional activity is evident, with fibrillation potentials and positive sharp waves seen. Motor unit potentials show decreased recruitment patterns and neuropathic changes in morphology.


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